New techniques of mass spectrometry employing tandem accelerators or cyclotrons at MeV energies have attained isotope abundance sensitivities near 10.sup.-16, many orders of magnitude better than the present 10.sup.-10 limits of conventional keV instruments. This improved capability, characterized by the term "ultra-sensitive mass spectrometry", has been applied to measurements of certain rare isotopes which are useful as chronometers and tracers. The list includes .sup.10 Be, .sup.14 C, .sup.26 AL, .sup.36 Cl and .sup.129 I.
The measurement of rare isotopes by ultra-sensitive mass spectrometry has several important advantages over techniques based on radioactive decay. The most significant of these is the gain in sensitivity, especially with the long-lived nuclides. For example, in the case of .sup.14 C a mass spectrometer with a detection efficiency of 10.sup.-3 would record 5.times.10.sup.4 14 C atoms per milligram of contemporary carbon from a biological source at a rate of one atom per second or more. In contrast the one mg of carbon (which is relatively short-lived amongst nuclides used as chronometers) would take eight years to generate 5.times.10.sup.4 beta decays. Other advantages are that mass spectrometry can, in addition, measure rare non-radioactive isotopes (as in the case of Ir and Pt), and that the ratios between abundant and rare isotopes are more readily obtained.
High energy mass spectrometry has demonstrated impressive capabilities for some of the lighter elements; abundances of 10.sup.-14 for.sup.10 Be, of 3.times.10.sup.-16 for .sup.14 C and 2.times.10.sup.-16 for .sup.36 Cl have been detected. However, because of isobaric and molecular interferences which become more difficult to eliminate for higher atomic number Z, high energy mass spectrometry cannot achieve comparable abundance sensitivites for the heavier elements. For example, the abundance sensitivity in the .sup.129 I measurements is limited to 10.sup.-11 due to isobaric interference by .sup.129 Xe. Another disadvantage of this technique is related to the difficulty in maintaining source and transmission stability, a problem which has thus far limited the accuracy of measurement. Also, in the case of the tandem accelerator, the technique can only be applied to atoms which form bound negative ions.
Recently there have been several proposals to employ laser techniques to achieve abundance sensitivities comparable to the ultra-sensitive high-energy mass spectrometer. Kudriavtsev et al, Applied Physics B 29,219 (1982), have proposed using collinear laser resonant multi-photon ionization of accelerated atoms. They describe two advantages to be gained by irradiating a several keV atomic beam along the direction of motion, namely:
1. The mass dependent velocity difference between two isotopes accelerated to the same energy produces a mass dependent Doppler shift which can improve the selectivity.
2. The velocity bunching that occurs during acceleration of the ions decreases the Doppler broadening which otherwise degrades the selectivity. For the case of .sup.26 Al vs. .sup.27 Al accelerated to 10 keV a selectivity of about 10.sup.-14 is calculated for the laser ionization process.
A second scheme proposes using Doppler-free resonance ionization as an ion source for a conventional mass spectrometer. This technique provides two or three stages of selection: one or two stages of laser ionization which can produce isotopic enhancements of greater than 1000 per stage in most cases, followed by mass analysis for an overall isotopic abundance sensitivity which could be as high as 10.sup.-13.
The laser-based schemes just described use selective laser ionization of the isotope of interest. Keller, et al Laser Focus 17, No. 10, 75(1981) have proposed the fluorescence detection of mass selected ions as they exit from a conventional mass spectrometer in order to discriminate against isobaric interference and enhance the overall abundance sensitivity. The projected selectivity for ions having resonance lines accessible to presently available lasers is greater than 10.sup.-15.